1. Field of the Invention
The present invention relates to a multiple-optical-axis photoelectric sensor in which an object detection area of multiple optical axes is formed by a plurality of light projecting elements constructing a projector and a plurality of light receiving elements constructing a photodetector. More particularly, the present invention relates to a multiple-optical-axis photoelectric sensor taking the measure against mutual interference.
2. Description of the Related Art
Generally, a multiple-optical-axis photoelectric sensor includes, as shown in FIG. 12, a projector 1A in which a plurality of light projecting elements 3A are aligned in line and a photodetector 2A in which a plurality of light receiving elements 4A paired with the light projecting elements 3A are aligned in line.
The projector 1A and the photodetector 2A are disposed with a proper distance so that the light projecting element 3A and the light receiving element 4A in a pair face each other in a one-to-one manner. The optical axis (shown by an alternate long and short dash line in the diagram) connecting the light projecting element 3A and the light receiving element 4A in a pair are parallel with each other. Between the projector 1A and the photodetector 2A, a two-dimensional object detection area S for detecting the presence or absence of an object is formed by a number of parallel optical axes.
Generally, a light emitting diode is used as the light projecting element, and a photodiode is used as the light receiving element. Each of the light projecting elements is allowed to perform light emitting operation at least once. To shorten time required to determine the presence or absence of an object, each of the light projecting elements can be allowed to perform the light emitting operation a plurality of times in a row.
Since the multiple-optical-axis photoelectric sensor of this kind can detect the presence or absence of an object in the object detection area S, it is used for, for example, a safety device of a pressing machine and the like. When a human body enters a dangerous area in the pressing machine, any of the optical axes is blocked by the human body (light shield state). When one or more optical axes in the light shield state exists, an object detection signal is set to the high level and an output is given to a controller of the pressing machine to urgently stop the operation of the pressing machine.
A failure in detection of an object by the multiple-optical-axis photoelectric sensor can cause a physical injury and it is very dangerous. One of factors of inducing a failure in detection of an object is disturbance light. When disturbance light enters the light receiving element of any of the optical axes blocked by the human body, the light shield state is not obtained, and the object detection signal is not set to the high level.
FIG. 13 is a diagram illustrating an installation example of a plurality of multiple-optical-axis photoelectric sensors A and B.
Referring to FIG. 13, to the photodetector 2A of the first multiple-optical-axis photoelectric sensor A, not only detection light “a” from the projector 1A but also detection light “b” from a projector 1B of the second multiple-optical-axis photoelectric sensor B as disturbance light is incident. In such a case, even if any of the optical axes is blocked by the human body in the first multiple-optical-axis photoelectric sensor A, if the detection light “b” of the second multiple-optical-axis photoelectric sensor B is incident as disturbance light on the light receiving element of the optical axis, the light shield state is not obtained. In the diagram, a photodetector 2B is shown in correspondence with the projector 1B of the second multiple-optical-axis photoelectric sensor B.
There are cases such that a plurality of multiple-optical-axis photoelectric sensors as described above are mounted close to each other in a site. In such cases, it is feared that so-called mutual interference occurs. In these days, various multiple-optical-axis photoelectric sensors taking the measure against such mutual interference are proposed.
For example, Japanese Patent Laid-Open No. 2002-217703 discloses a multiple-optical-axis photoelectric sensor in which disturbance light synchronized with a timing of light generating operation of each light projecting element is detected around the timing of light generating operation of the light projecting element. When disturbance light is detected, the timing of the light generating operation of the light projecting element is shifted, thereby preventing a failure in detection of an object caused by the disturbance light synchronized with the timing of the light generating operation of the light projecting element.
However, shifting of the timing of the light generating operation of each light projecting element, that is, the light projection/reception cycle disclosed in the publication is advancing or delaying of the timing for a predetermined period.
In some cases, the timing has to be shifted a plurality of times in order to be adjusted to a proper timing, and a failure in detection may occur during the time of shifting. That is, desirably, it is proper to adjust the light projection/reception cycle in accordance with a state where mutual interference is detected.